Pulsed laser micro-machining techniques using mask projection methods are now widely used for the creation of miniature structures in both massive and thin substrates. Micro-electro-mechanical systems (or ‘MEMS’) are frequently prototyped by ‘excimer’ laser ablation of polymer substrates using motion of the substrate and/or a mask to create complex 2.5D and 3D structures. Typically sub-micron thick inorganic, metallic and organic films incorporated in, for example, solar panels, sensors and display devices are often patterned using mask projection methods with both solid state and gas laser sources. Over many years such processes have become well established as production techniques. Improvements have mainly been directed to enhancements in laser drive technology rather than changes to basic mask projection, beam handling and motion control techniques.
In a paper ‘New Techniques for Laser Micro-machining MEMS Devices’ in Volume 4760 of the Proceedings of The International Society for Optical Engineering there are described two pulsed laser micro machining techniques termed ‘Synchronised Image Scanning’ (‘SIS’) and ‘Bow Tie Scanning’ (‘BTS’). These techniques take advantage of the improvements made in speed and accuracy of modern stage and galvanometer mirror scanner systems. Both SIS and BTS allow major improvements to be made in the accuracy, speed and efficiency with which large area complex repeating arrays of miniature 2D and 3D patterns can be created by laser ablation. The techniques are readily used in the laser manufacturing of devices such as super-long ink jet printer nozzles (e.g. page-wide arrays), micro lens arrays, display enhancement films and plasma display panels (PDPs).
For precision laser micro machining it is essential to control accurately the distance between the focusing or imaging lens to the surface being machined. Typically the distance needs to be maintained within the depth of focus of the lens system.
In attempting to provide for accurate control of the distance between the projection lens and a substrate to be ablated by micro machining two methods are already known. Neither is trouble-free.                Firstly by mounting a substrate to be machined on a transport system and making use of laser reflection tracking to give information about the substrate position. Laser reflection tracking can give rise to difficulties arising from changes in transmission characteristics in the substrate material. Transport systems for the substrate can give rise to lagging or leading positional errors caused by feedback delays. Multi surface reflections or reflectivity changes can lead to locating errors.        Secondly by using ultra flat chucks to carry the substrate. Ultra flat chucks are unable to take into account variations in thickness of the substrate. Some types of substrate allow the passage of a laser beam that can lead to heating, if not damaging, of the chuck. Such chucks are inherently expensive to manufacture to sufficiently tight tolerances.        
In considering a typical workpiece for laser micro-machining in the present context it is convenient to consider it in three dimensional rectilinear terms with a length measured along an X-axis, a width measured along an Y-axis and a thickness measured along a Z-axis. The thickness, measured along the Z-axis, is usually orders of magnitude less than the length, measured along the X-axis, or its width, measured along the Y-axis. Hereinafter such a work-piece is referred to as being ‘of the type described’. However the present invention is not limited to a workpiece of these proportions and can be adapted to deal with work pieces of other shapes.
In considering a micro-machining process and the required position of a laser beam relative to a workpiece it is convenient to use the term ‘datum position’. In the case of a focussed beam the point of focus represents the datum position. In the case of an image formed when a laser beam has passed through a mask then the location of the image formed by way of the laser beam is taken to be the datum position. In either case, to provide for accurate ablation or heating of a substrate, the datum position needs to be maintained in a fixed relationship relative to the surface of the substrate. In what follows the term ‘datum position’ should be interpreted appropriately for the laser optical system under consideration.
The term ‘micro-machining’ should be taken to include not only an ablation or image forming process but also heating of the workpiece at some position between the outside surfaces of the workpiece.